Wednesday, September 30th, 2020

Research Vignette: Asynchrony/Synchrony vs. Mobile/Stationary

by Eli Gafni (UCLA)

In the Simons Institute program on Proofs, Consensus, and Decentralizing Society, systems like Stellar reminded us of the anomaly that results from introducing cryptography into distributed computing.

The research I embarked on was to find conditions under which the anomaly is eliminated.

In synchronous systems with processors using signatures that may behave in a Byzantine malicious manner, consensus can be reached as long as at most half of the processors are engaged in the malicious behavior. In contrast, when the system is asynchronous but eventually settles on synchronous behavior, consensus can be reached only under the more stringent condition that at most a third of processors rather than at most half are malicious.

This anomaly that synchronous systems and eventually synchronous systems do not have the same capability in reaching consensus is exhibited only with the introduction of cryptography signatures.

We resolve this anomaly by proposing that the definition of asynchrony, while practically pleasing, should be substituted by the more elegant, appealing mathematical definition of viewing asynchrony as the traditional synchronous system, but with the faulty behavior exhibited by different sets from round to round, giving rise to the pair mobile/stationary rather than asynchronous/synchronous.

This shift raises two questions: one about the meaning of mobile Byzantine and the other about the meaning of possibly all signatures forged over the rounds.

We resolve the first question by attributing the faulty behavior solely to the communication subsystem, considering processors to always behave correctly. We resolve the second question by abstracting signatures as just a constraint on which forwarded messages can be forged and which cannot. (We leave open the question of implementing the constraint.)

With these resolutions, a stationary system with signature constraint can reach consensus in the face of malicious faults as long as no more than half of processors fail in a round. The notion of eventual stationarity is now the elegant notion that the mobility of the system freezes. The system was hot and is now cooled. This supports the observation that, mathematically, the correct pair to consider is mobile/stationary rather than asynchrony/synchrony.